1. Field of the Invention
The present invention provides a composition suitable for primary anticorrosive treatment of the surface of a non-ferrous metal, a process for producing said composition, a process for treating the surface of non-ferrous metal with said composition, and non-ferrous metal of which surface is treated with said composition.
2. Related Arts
The surface of a non-ferrous metal substrate, especially a non-ferrous metal plated steel substrate has a very beautiful appearance and shows a relatively good corrosion-resistance. Therefore, materials having non-ferrous metal surface are widely used for manufacturing, such as, auto mobile parts, electronic appliances, heat exchanger part roof, and the like. Although they show relatively good resistance against corrosion, they still suffered from corrosion problem depending on storage condition. Especially, aluminum containing non-ferrous metal materials tend to be suffered from corrosion problem such as blackening or white rust. In order to avoid such problems, the surface of a non-ferrous metal substrate is generally treated with an anticorrosive composition before being processed into a product. Such treatment has known as anticorrosive primary treatment. A method of typical anticorrosive primary treatment is known as chromate treatment, which comprises a step of coating the surface with a chromate film by applying a chromate anticorrosive treatment composition consisting mainly of chromic acid and water.
A conventional chromate primary treatment may be practiced using an aqueous chromate solution comprising partially reduced chromic acid, that is, an aqueous solution containing both Cr.sup.3+ and Cr.sup.6+. Such the conventional chromate anticorrosive treatment composition can be obtained by reducing a part of hexavalent chromium to trivalent chromium with a reducing agent. Preferred reduction ratio may be determined based on other ingredients of the chromate anticorrosive treatment composition, treatment condition, type of the non-ferrous metal substrate to be treated, and the like. Generally, the chromate anticorrosive treatment composition of which reduction ratio, or a degree of Cr.sup.6+ reduced to Cr.sup.3+, is from about 30% to about 70% has been used for primary treatment. That is, the typical chromate anticorrosive treatment composition has the Cr.sup.6+ /Cr.sup.3+ weight ratio of between 3/7 and 7/3.
The art knows a variety of reducing agents may be employed in reducing hexavalent chromium to trivalent partially, including organic compounds such as starch, saccharides and alcohols, and inorganic compounds such as hydrazine and hypochlorous acid.
Although this type of primary treatment is popular in the art, it is still difficult to control the reduction rate within the preferred range and to keep the ratio for a long time because of the reducing agent remained in the composition. When a chromate anticorrosive treatment composition of which reduction ratio is low, or the weight ratio of Cr.sup.3+ /Cr.sup.6+ is low, is used for primary treatment, the obtained chromate coating film will appear yellow. In order to avoid this yellowing problem, the thickness of the chromate film must be very thin and highly uniform. Further, since Cr.sup.6+ is soluble in water, the obtained chromate coating film easily loses its adhesivity to a secondary paint under a corrosion inducing condition, and therefore, the corrosion resistance ability of the obtained product of which the primary treated surface being coated with the secondary paint becomes poor.
To the contrary, when the reduction ratio of hexavalent chromium in the treatment composition is high, for example, more than 70%, the composition may set to gel with time, and its corrosion resistance becomes poor. Further, the chromate coating film obtained by the conventional method usually appears an interference pattern. That is, the arts in the field acknowledged that if a chromate anticorrosive treatment composition of which reduction ratio of hexavalent chromium is 100% were used, there would be no possibility to obtain a good result.